In an internal combustion engine wherein a pair of intake valves and a pair of exhaust valves are disposed in radial arrangement, a rocker shaft of the intake side and a rocker shaft of the exhaust side, supporting rocker arms, are heretofore disposed at the same height, the rocker arms pressing the intake valves and the exhaust valves, and holders supporting these rocker shafts on the intake side and the exhaust side are arranged also at the same height (refer to Patent Document 1).
In general, the diameter of the exhaust valves is smaller than diameter of the intake valves. However, prior art internal combustion engines are not designed in consideration of this. Therefore, an optimum arrangement of the valve drive mechanism and an optimum setting of the angle of radial arrangement of the intake valves and the exhaust valves as well as miniaturization of the engine head portion have been underlying problems.